This invention relates to a switched-mode power supply circuit for converting a d.c. input voltage into a d.c. output voltage, including a controllable power switch arranged in series with the primary winding of a transformer. The series arrangement thus formed is coupled to the input voltage terminals. The transformer has a secondary winding to which a rectifier is coupled for providing the output voltage. The primary winding forms part of a resonant circuit when the switch and rectifier pass no current. A drive circuit including a delay network is connected to a control electrode of the switch for each time initiating turn-off of the switch, whereafter the voltage across the switch increases to a given value and substantially maintains said value. The said voltage then decreases in accordance with an oscillation at the resonance frequency. The drive circuit each time initiates turn-on of the switch, whereafter the voltage across the switch is substantially zero.
A power supply circuit of this type is known from U.S. Pat. No. 4,183,080. It is a self-oscillating circuit in which the drive circuit for the power switch, which is a switching transistor, includes a secondary winding of the transformer, which secondary winding is coupled with the primary winding to provide a positive feedback. A network for rendering the transistor conductive and for maintaining this state is incorporated between the said secondary winding and the base of the transistor. For turning off the transistor, the drive circuit includes a delay network connected to the same winding and a drive transistor whose main path is arranged in parallel with the base/emitter junction of the switching transistor. At an instant determined by the delay network the drive transistor begins to conduct so that the switching transistor is rapidly rendered non-conductive. In this manner the turn-off losses of this transistor are reduced, more specifically because of the short duration of the turn-off process. The conducting time of the drive transistor is very short. Together with parasitic capacitances the primary winding constitutes a resonant circuit.